The proposed research would examine and assess properties of short-interval timing and temporal resolution at different phases of the circadian activity/rest rhythm in the pigeon. The central hypothesis is that the biological time-measurement device - the timer or clock governing temporal estimation and discrimination - runs at different speeds and with concomitant changing resolution as a function of circadian phase. The reference procedure (Gibbon and Church, 1981) should be able to distinguish effects resulting from different speeds of the internal clock from those attributable to motivational or other variance-inducing factors. Preliminary investigations on free-running procedures in the pigeon will be required to establish the feasibility of the preparation. These experiments are of interest in their own right in informing our understanding of free-running rhythms of feeding and drinking with and without entrainment to food restriction schedules.